The production of silicon integrated systems, whether they be power or processing systems, is increasingly carried out with differential and variable reference impedance structures for analog portions. External components remain essentially a system of common mode type components with 50Ω reference impedance.
The link between a balanced transmission line and an unbalanced transmission line cannot be produced without an appropriate electrical circuit. This transition is provided by a transformer of the balanced-unbalanced type, also called “balun.” A balun converts, for example, a signal of the common mode type into a signal of the differential mode type, and vice versa, and provides impedance transformations. One of the main electrical characteristics of a balun is its insertion loss, which must be as low as possible.
Baluns can also be used, for example, in receiving and transmission circuits of wireless communication systems for the design of differential circuits such as amplifiers, mixers, oscillators and antenna systems. Baluns can be made with transmission lines such as Lange couplers, couplers of the circle-shaped-type commonly known to those skilled in the art as “rat-races”, or Marchand couplers or else with stacked or coplanar inductors.
In the transmission and reception circuits of wireless communication systems, the impedance on the differential side may be low, typically of the order of 10 to 20 Ohms for a low-noise amplifier, while the impedance on the common mode side, that is to say on the antenna side, is, as indicated above, usually of the order of 50 Ohms. Thus, there is the need to have a transformation ratio higher than 2, which is particularly complicated to achieve.
Moreover, in transmission, the power amplifier should be supplied with a current of the order of a few hundreds of milliamperes. And, if it is desired to supply the power amplifier via the transformer (balun), there is a resulting impact on the performance of this balun.